Water systems which obtain their water by pumping from underground water tables using wells can have numerous water quality problems. It is typical to find that ground water has high amounts of dissolved minerals which come from the rocks within the aquifers. Further is it quite common to find nuisance and even toxic metals dissolved into the water. Treating this ground water so that the resulting water is appealing or safe to use can be a challenging pursuit.
Most of the minerals and metals that are dissolved in ground water are ionic, that is, the molecules are disassociated atoms in solution. These ions are quite small, ranging from 3-20 angstrom units or 0.0003 to 0.0020 microns in size. Filtration of ions with conventional filtering technologies is almost impossible, leaving only the more expensive treatment options such as reverse osmosis, deionization or distillation. It is easy to see the minerals which are dissolved in water as they leave white or colored powders and deposits when the water is evaporated. If the deposit is easily dissolved by adding fresh water, the deposit is soft. If the deposit does not readily dissolve in water again—it is considered hard. Water with hard minerals dissolved into it is problematic to water systems, particularly with water heating systems, as hard water tends to build up scale and thick deposits of these precipitated insoluble minerals and damage or occlude the piping, heat exchangers, or tanks etc. Soft waters do not experience this kind of problem.
To treat minerals, it is quite common to exchange one type of mineral for another by using an electrostatic resin. For example, a typical water softener can exchange calcium and magnesium carbonates (hardness) for sodium chloride (softness). In this process, the water softener has a large bed of resin beads composed of quaternary ammonium cations. Each bead has an electrostatic charge such that the bead can be coated to hold an amount of common salts, with sodium chloride being among the most common. This anionic ion-exchange resin has an affinity to certain minerals and prefers “hard” minerals more than the “soft” minerals. In use, water containing hard minerals passes by these resin beads and the hard minerals stick to the resin while the soft minerals are displaced from the resin. The process continues until there are no remaining soft minerals on the beads. When the beads are saturated with hard minerals, even these hard minerals begin to escape such that “hard” water beings exiting the water softener. The process is reversed by flooding the spent resin with a saturated solution of soft minerals until all of the hard minerals are removed in a process called brining. After a water rinse, the resin bed is ready to exchange soft for hard minerals again. Water softening does not remove minerals but instead exchanges problematic hard minerals for soft minerals.
Dissolved metals pose a similar problem. These metals are normally dissolved into the ground water in the absence of oxygen so they are transparent. Iron is a common metal contaminant and is usually found as clear-water iron or ferrous iron. Iron in the ferrous or non-oxidized form is not able to be filtered with conventional filtering technologies and again requires the more expensive methods for removal such as, for example, reverse osmosis, deionization or distillation. Iron can also be removed from water using a chemically based removal system, such as potassium permanganate and greens and filtration systems. However, if ferrous iron is exposed to oxygen, the resulting oxidized iron precipitates into a larger, suspended molecule, which can then be filtered using conventional filtration techniques including a large variety of common media filters using sand or other mineral particulates being the most common. While iron and manganese tend to be among the most common dissolved metals, arsenic, chromium, and other highly toxic metals are also able to be filtered with conventional filtering technologies when oxidized.
Most systems that are in use today to remove iron and manganese use air as the source of oxygen. Air contains about 78% nitrogen and 21% oxygen—with the balance comprising carbon dioxide and other trace gases. Air is introduced into the contaminated water using a variety of methods and given enough time, the dissolved metals will oxidize and there precipitate for removal using conventional filtration techniques.
Most commercial iron removal systems require long retention times to adequately oxidize iron and manganese, and are not normally able to treat some of the other more toxic metals. Further, the added minerals in these waters are also precipitated and foul the piping, tanks, valves—which reduces the life of all of the components. In addition, water containing dissolved iron is often plagued by iron-eating bacteria. This type of biological contaminant is not normally toxic, but can cause significant problems in water and water treatment equipment by forming thick impervious biofilms. Evidence of the presence of iron-eating bacteria can include orange and blood-stained clothing, toilets, showers, as well as a foul odor.
There have been recent advances in the technology of aqueous metallic oxidation by using electrolysis. U.S. Pat. No. 6,689,262 discloses a method to produce pure micro-bubbles of oxygen in water, and U.S. Patent Applications 2006/0150491 and 2004/0118701 teach methods of utilizing this technology to treat water in flow-through devices. In general, these techniques use electricity and titanium electrodes to convert water into microbubbles of pure oxygen so as to cause dissolved aqueous metallic contaminants to be oxidized for subsequent removal with suitable barrier filtration technologies. These micro bubbles of oxygen rapidly and effectively convert dissolved metals into metallic oxides suitable for conventional filtering. Flow-through chambers are constructed to house various arrays of anodes and cathodes and allow the efficient generation of micro bubbles of pure oxygen. These very small bubbles rapidly dissolve into water at a very high rate and can often super-saturate water. These devices are so effective at oxidizing water electrolytically that they will precipitate metals and minerals to an excess.
Unfortunately, electrolytic water treatment systems are subject to fouling and operational disruptions caused by the formation and deposit of precipitated minerals and metallic oxides on the electrodes. These precipitates build rapidly on electrodes and surrounding surfaces until they become occluded and rendered electrically impotent. In response, prior systems have included methods for alternating the polarity of these electrodes periodically such that a partial reversal of the scaling process can take place. Any electrode acting as an anode will evolve hydrogen-based acidic water (H+) near the electrode surface as it releases oxygen bubbles. This acidic water film can help to remove surface deposits which were precipitated during the previous cathodic operation interval. At the opposing electrode, or cathode, water is converted into a basic or alkaline species with an excess of hydroxide molecules (OH−). In this environment, hydrogen is evolved while scale is precipitated on the anode's metal surface. The reversal of the electrical polarity will help forestall the eventual degradation of the electrodes but as they are used, they will decay to a point which renders them ineffective. By treating the titanium electrode with a catalytic coating such as iridium, platinum, rhodium, and ruthenium, various anodic and cathodic reactions can be optimized to produce cleaner longer-lasting electrodes and yield of preferred evolved gases.
Given the state-of-the-art in electrode technology materials, using the optimum polarity reversal timing schemes, and even employing intelligent current and voltage control, these electrolytic chambers have a relatively short life—even thought they are very efficient at precipitating metallic and mineral contaminants. As such, it would be advantageous to improve on current electrolytic treatment systems so as to increase their effective life.